1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly, to a backlight unit for a liquid crystal display device.
2. Discussion of the Related Art
With the rapid development of information technology, flat panel display (FPD) devices having advantages of thin thicknesses, light weights and low power consumption, have been developed and have replaced cathode ray tubes (CRTs). The FPD devices include liquid crystal display (LCD) devices, plasma display panels (PDPs), electroluminescent display (ELD) devices and field emission display (FED) devices.
Among these devices, liquid crystal display (LCD) devices have been widely used for monitors of notebook computers, monitors for personal computers and televisions because the LCD devices are excellent at displaying moving images and have a relatively high contrast ratio.
LCD devices are not self-luminescent and require an additional light source. By disposing a backlight unit at the rear side of a liquid crystal panel to emit light into the liquid crystal panel, discernible images can be displayed. Backlight units may include cold cathode fluorescent lamps (CCFLs), external electrode fluorescent lamps (EEFLs) or light-emitting diodes (LEDs) as a light source. 
Backlight units are classified as edge type or direct type according to the position of the light source with respect to a display panel. In edge-type backlight units, one or a pair of lamps are disposed at one side or at each of two sides of a light guide panel of a backlight unit. In direct-type backlight units a plurality of lamps are disposed directly under the display panel.
FIG. 1 is a cross-sectional view illustrating a liquid crystal display (LCD) device including a direct-type backlight unit according to the related art. The related art LCD device includes a liquid crystal panel 10, a backlight unit 50, a support main 60, a top cover 70 and a cover bottom 80. The liquid crystal panel 10 includes upper and lower substrates 13 and 15. A printed circuit board (not shown) is attached at a side of the liquid crystal panel 10. The backlight unit 50 is disposed under the liquid crystal panel 10. The support main 60 surrounds side surfaces of the liquid crystal panel 10 and the backlight unit 50. The top cover 70 covers edges of a front surface of the liquid crystal panel 10, and the cover bottom 80 covers a rear surface of the backlight unit 50. The top cover 70 and the cover bottom 80 are combined with the support main 60 to thereby constitute one-united body.
The backlight unit 50 includes a reflection sheet 40, a plurality of lamps 22, a diffuser plate 30, and a plurality of optical sheets 20. The plurality of lamps 22 are disposed over the reflection sheet 40, and the diffuser plate 30 and the plurality of optical sheets 20 are sequentially disposed over the plurality of lamps 22.
The plurality of optical sheets 20 includes a diffuser sheet and at least one light-concentrating sheet. The plurality of optical sheets 20 will be described in more detail with reference to FIG. 2.
FIG. 2 is a view schematically illustrating the diffuser plate and the plurality of optical sheets of FIG. 1. As shown in FIG. 2, the plurality of optical sheets 20 including a  diffuser sheet 24 and a light-concentrating sheet 26 are sequentially disposed over the diffuser plate 30.
The diffuser plate 30 may be formed of a transparent acrylic resin, polymethylmethacrylate (PMMA) or thermoplastic polyethylene terephthalate (PET) including irregular particles therein. The diffuser plate 30 may have various haze properties according to light uniformity. The diffuser plate 30 directs light emitted from the plurality of lamps 22 of FIG. 1 towards the liquid crystal panel 10 of FIG. 1 and diffuses light such that the light is incident on the liquid crystal panel 10 of FIG. 1 within a relatively wide range of angles.
The diffuser sheet 24 positioned over the diffuser plate 30, generally, is composed of a base film of PET and acrylic resin layers on both sides of the base film, each of which includes light-diffusing materials such as beads. The diffuser sheet 24 diffuses light passing through the diffuser plate 30, so that there may be no patched portion due to partially centralized light, and the diffuser sheet 24 directs the light towards the light-concentrating sheet 26.
The light-concentrating sheet 26 includes a base film of PET and prism patterns 28 regularly arranged on an upper surface of the base film. Generally, the prism patterns 28 reflect or concentrate light.
FIG. 3 is a view illustrating paths of light passing through the light-concentrating sheet of FIG. 2.
As shown in FIG. 3, light incident on the light-concentrating sheet 26 may have an angle of about 5 degrees with respect to the normal to a lower surface of the light-concentrating sheet 26. Light incident on a light-concentrating region A is refracted by the prism patterns 28 and is concentrated toward the liquid crystal panel 10 of FIG. 1 to thereby  cause an increase in brightness. Light incident on a total reflection region B is totally reflected downward at surfaces of the prism patterns 28.
The downward reflected light is re-reflected by the reflection sheet 40 of FIG. 1 and is re-supplied to the light-concentrating sheet 26. Therefore, a loss of light is minimized by the circulation of light.
However, in addition to the concentration and reflection of light in the light-concentrating sheet 26, there is a loss of light due to side lobes. This causes a decrease in light efficiency and a lowering in viewing angle properties of the display.
More particularly, in the figure, a region designated by C means a region where there occur side lobes, wherein the region C is referred to as a side lobe region hereinafter.
FIG. 4 is a view of illustrating angular distribution of brightness of light passing through the light-concentrating sheet of FIG. 2.
As shown in FIG. 4, there are bright areas at both sides with respect to a symmetrical point at a center, that is, side lobe regions C. Light incident on the side lobe region C is refracted at the surface of the prism patterns 28 toward an undesired direction and is not incident on the liquid crystal panel 10 of FIG. 1 to thereby be wasted. In other words, there is light leakage in the side lobe regions C.
To solve the problems described above, a reflective polarizing sheet or a diffusion film for recycling light may be further disposed over the light-concentrating sheet to decreased or removed the side lobes to thereby improve a light efficiency and viewing angle properties. However, there still exist the side lobes.